The primary specific aim of this study is to examine the genetic and environmental influences underlying the human behavioral traits of aggression and impulsivity in a sample of population-based twin subjects. While these dimensions of behavior have previously been studied in twin samples, no study has: a) examined both behavioral dimensions in the same subjects, b) utilized different and multiple modes of assessment (e.g., clinical and laboratory based assessments), c) examined the relevance of specific measures reflective of environmental factors (e.g., "experienced" and "witnessed" aggression), d) examined the nature of differences in genetic and environmental influence as a function of gender or, e) examined measures of 5-HT and aggression in twin subjects. In this study, both aggression and impulsivity will be examined using a variety of self-report (clinical and personality based) and laboratory assessments of aggression and impulsivity. In Phase I of this project, twins will be administered two (2) self-report measures each for aggression and impulsivity. One measure each will assess history of actual aggressive and impulsive behaviors. In addition, one measure each will constitute a personality assessment of aggressive and impulsive tendencies. In addition, other questionnaires will be administrated to assess selected variables relevant to environmental factors putatively involved in impulsive aggressive behavior (e.g., experience of aggression, witnessed aggression, perception of parenting practices). Mixed-Gender DZ twin pairs will also be studied in this phase so that analysis of potential gender differences in aggression can be examined. In Phase II, 420 twin pairs will be studied in our laboratory at The University of Chicago using two sets of analogue laboratory assessments of aggression (Point Subtraction Aggression Paradigm: PSAP, and Taylor Aggression Paradigm: TAP) and impulsivity (Cherek Impulsivity Paradigm: CIP and the Immediate and Delayed Memory Task: IMT/DMT). Finally, structural (5-HT Transporter Binding), functional (5-HT Uptake), and genetic (DNA Polymorphism for the 5-HT Transporter) measures of the Platelet Serotonin Transporter will be assessed so that the genetic correlation between indices relevant to serotonin and aggression/impulsivity can be compared with the inverse phenotypic correlation previously established, between these two sets of variables, over the past two decades of research.